Computed tomography (abbreviated CT) has opened whole new applications for the general subject of imaging employing penetrating radiant energy. Prior to computed tomography, imaging employing penetrating radiant energy was substantially limited to production of radiant energy shadow graphs (sometimes called a projection radiograph), i.e. the typical X-ray image. Such images suffered from a number of drawbacks which limited the utility of the resulting product. Because the projection radiograph provided only a shadow image, distinct objects lying in the path between the source of penetrating radiant energy and a film plane produced shadows of those objects which were superimposed one on another. This made it difficult for one interpreting such an image to distinguish one object from another, to delineate the shape of the different objects, to determine their relative densities and/or relative positions.
In computed tomography, no film is used, instead one or more radiant energy detectors are used. A source of penetrating radiant energy is directed at the location of the detector or detectors, and the object to be imaged is moved relative to the source/detector arrangement. Depending on the configuration of the source of penetrating radiant energy and the configuration of the detector and/or detectors, that motion may be limited to simple rotation, or it may be a more complicated motion consisting of translation and rotation. Regardless of these variables, a requirement for computed tomography is the production of a plurality of "views" of the object being imaged. Each view consists of absorption data describing the transmissivity of the object being imaged at a single angle relative to the source/detector arrangement. After obtaining similar absorption data for a plurality of views, the entire ensemble of absorption data is convolved and back-projected so as to produce an image corresponding to a cross section of the object being imaged taken at the plane of the object through which the penetrating radiant energy is directed. Different cross sections, or slices, are produced by repeating the foregoing steps with the illumination directed at different cross sections of the object to be imaged.
By far the most widespread application of computed tomography is in the medical field wherein images useful in diagnostic procedures are employed. As can be easily understood, the design of computed tomography equipment for medical applications is selected so as to optimize the resulting image. To this end, for example, the illuminating source is typically an X-ray source of energy in the range of 150 kilovolts.
Computed tomography, however, is not limited to the medical field and has wide applicability in the general field of non-destructive testing. There are many objects in which a cross section image would be very valuable. For purposes of describing this invention, a solid fuel rocket motor will be taken as exemplary of a wide variety of objects, a cross section of which would be very desirable. These objects all have in common a number of characteristics, for example they are:
1. much more dense than internal human organs, areal density may be measured in hundreds of grams per square centimeter,
2. they are not restricted to the same size scale as human organs, although it may be desirable to be able to view minute portions (measured on the order of thousandths of an inch or less) in an object with a diameter measured in the range of many feet.
Although imaging such objects using penetrating radiant energy has, in the past, been employed, see for example Heffan et al U.S. Pat. Nos. 3,766,387; Mauch et al 3,894,234 (related to rocket motors); Stewart 3,992,627 (related to gas turbine engines); Kenney et al 3,769,507 (related to optically opaque objects including components of the human body as well as metal castings, pipes, plates, complex mechanical devices, etc.); and Cherry 3,008,049, to the best of applicants' knowledge, computed tomography has not been employed with relatively dense and large objects such as rocket motors so as to produce images which can be used to delineate small defects such as cracks, voids and separations with dimensions measured on the scale of thousandths of an inch located anywhere within the motor.
As indicated in Kenney et al, the key to conventional radiography is differential absorption of radiation where variations in thickness, density and chemical composition provide differing attenuation for the penetrating radiant energy. Kenney indicates, however, that the absence of significant density differences, for example a hairline crack in a metal casing, etc., makes it almost impossible to successfully detect such discontinuities. Kenny also indicates that in conventional radiography, scattered radiation is considered undesirable because it results in fogging and poor definition of the radiograph. This is also true in computed tomography wherein scattering degrades the view and renders accurate back projection impossible. While Kenney indicates the effects of scattered radiation are minimized by the use of lead screens or diaphragms, the use of such screens and/or diaphragms becomes less and less attractive as the energy of the penetrating radiant energy increases, because as the energy increases a given quantity of lead has less and less of an effect. For other examples of collimation, see Wilson, Jr., U.S. Pat. No. 3,151,245; Ashe et al U.S. Pat. No. 4,096,389; and Wagner U.S. Pat. No. 4,286,156.
It is therefore one object of the present invention to provide an apparatus for computed tomography on highly opaque objects of significant size wherein it is desired to image portions of the object which may be very much smaller than the thickness of any material which can collimate the illuminating energy. It is another object of the present invention to provide an apparatus for producing computed tomography using high energy illumination (one million) electron volts or more).